Announcing Extension.js 4.0.0

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The one breaking change is Node.js

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What ships in v4

• Classic multi-file content scripts work in development. When one content_scripts.js array lists several plain files, Extension.js concatenates them into the single shared scope the browser would give them, registers every file as a build dependency so any save rebuilds, and emits a source map that points at your real file and line instead of an inlined blob.
• Faster Safari development. extension dev --browser=safari resyncs in the background instead of blocking on a full Xcode build every save, and a burst of saves collapses into one rebuild against the newest output.
• No leaked browsers. A dev session that exits on its own now shuts the browser down through a shared teardown path, so Chrome and Firefox processes stop lingering after you are finished.
• Six more browsers by name: Brave, Opera, Vivaldi, Yandex, Waterfox, and LibreWolf, alongside the chromium-based and gecko-based targets for custom binaries. Forks inherit the chrome: and firefox: manifest keys of their engine family, so a fork target does not need its own prefix.
• Reload without a launched browser. extension dev --no-browser now reloads content scripts too, through the same service worker path a launched browser uses, which makes it a real option for continuous integration (CI), containers, and remote machines.
• Profile control that holds. profile: false, copyFromProfile, and keepProfileChanges are honored from config through to both the Chromium and Firefox launchers, typed on BrowserConfig, and a kept profile is no longer clobbered on later runs.
• Two dev sessions stop fighting over a debugging port, because the Chrome DevTools Protocol (CDP) and Remote Debugging Protocol (RDP) ports now resolve per browser instance.
• Louder, cheaper failures. Compilation errors that prevent output exit non-zero, a Manifest V2 build against a Chromium target warns, a missing CSS url() asset warns and passes the URL through instead of failing the build, and a missing optional dependency prints a package-manager aware install hint instead of a raw JSON blob.
• Windows content-script paths are canonicalized, so loader matching and content-script wrapping behave the same as on macOS and Linux.

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The day-one flow has not changed

npx extension@latest create my-extension
cd my-extension
npm run dev

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Upgrade notes

• move to Node.js 22.12 or newer, then update the package to latest,
• keep your extension.config.js as it is, since there are no API changes to follow,
• if you were working around multi-file content scripts by inlining them into one file, you can split them back out.

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What landed after 4.0

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Thanks